This invention relates to metal compositions having improved mechanical properties and to a method for making the metal compositions. More particularly, this invention relates to a method for making such compositions wherein a portion of the metal sample is removed as liquid during heating at elevated temperature and pressure followed by heat treating the remaining solid.
Present methods for obtaining high strength metal alloys all depend on producing a very fine distribution of the interdenritic secondary phases present in the alloy. This fine distribution is necessary so that the dissolution of the second phases into the primary phases can be accomplished in a relatively short time.
All cast alloys possess significant amounts of these secondary phases which result from "nonequilibrium" solidification. The presence of these secondary phases limits the compositions of the alloys that can be practically cast and worked, and the properties of these alloys. For example, significant increases in strength of 7000 series aluminum alloys could be achieved by increasing the alloy content above that of 7075 alloy or its newer modifications. However, when such increases are made in ingots, the amount of secondary phases increases to a point that ingot-working is difficult or impossible, ingot cracking is a problem and the secondary phases are not fully solutionized within practically obtainable cycles.
Secondary phases also result from impurities present, such as iron in 7000 series alloys and these are not in general eliminated by heat treatment in processes employed up to this time. Secondary phases are also well known to limit the mechanical properties of ferrous and superalloys, examples being the sulfides in steel and the alloy eutectic in .gamma.' strengthened superalloys. If these alloys could be produced with a structure exactly like that now achieved, but without precipitation of this secondary phase at the end of solidification, significant increases in room temperature and elevated temperature properties could be obtained.
There are several processes used at present to produce a fine distribution of secondary phases. Processes such as atomization (production of alloys from rapidly solidified powders and compaction), mechanical working of conventional ingots, and the use of chill molds are all employed to obtain a fine distribution of secondary phases in the metal alloys. However, these processes are difficult and expensive and often unreliable. It is thus costly at present to obtain a high alloy content metal free of second phases, limiting the commercial use of these metals to highly specialized applications.
Fractional crystallization as a means for purifying metal alloys is well known. The available purification procedures are based upon the fact they when an alloy which solidifies over a temperature range is taken to a temperature within its liquid-solid range, the solid phase has a composition different from the liquid phase. The average composition of both the liquid and solid portions of the metal alloy also depends upon the temperature of the liquid-solid mixture. By controlling the temperature of the system, the impurity to be removed, which in this case is in the form of secondary phases which are deleterious to the metal compositions, can be isolated in either the solid or liquid phase. Thereafter, the solid and liquid phases are separated by physical means and the purer fraction is recovered.
A wide variety of physical separation means have been proposed which, combined with fractional crystallization, are designed to recover pure metal alloys or metals. For example, centrifugal filtration has been proposed wherein the liquid-solid mixture is subjected to centrifugal forces up to about 2,500 g and the liquid under this force is passed through a filter comprising all or a portion of the wall of the container housing the mixture and the liquid phase to accumulate within the central portion of the container. The liquid then is recovered selectively, such as by decantation. Also, the process disclosed in U.S. Pat. No. 3,211,547 to Jarrett et al, issued Oct. 12, 1965 includes a step of forming crystals on the top surface of a liquid-solid aluminum composition allowing the crystals to settle and then compressing up to one-half of the surface area with tampers. The liquid then is recovered. Also, it has been proposed in British Pat. No. 508,925 to heat a metal alloy to form a liquid-solid mixture and then subject the mixture to a pressure of at least 500 atmospheres in order to force the liquid through a filter to leave a solid.
While these separation processes are capable of increasing the purity of metal compositions, they are undesirable in that a substantial portion of the liquid remains unseparated from the solid and the "efficiency" of the process is low, i.e., that often the separation process must be repeated a number of times to obtain the desired purification. Furthermore, when the alloy contains a small but excessive concentration of material capable of forming secondary phases, these processes provide little, if any, improvement in product purity and prevent the alloy from developing high strength.
The process provided herein overcomes these objections and provides efficient economical means of forming metal alloys having improved mechanical properties resulting from substantially complete dissolution of material forming interdendritic secondary phases into the primary phase material or its elimination by exudation out of the sample.